The present invention relates generally to the technical field of control of electric motors, and in particular to the control of synchronous permanent magnet electric motors.
A synchronous permanent magnet motor comprises a rotor having one or more permanent magnets and also a stator comprising windings in which currents phase-shifted in relation to one another circulate so as to create a rotary magnetic field in the motor, driving the rotor in rotation. Because the frequency of rotation of the rotor is equal to the frequency of the currents circulating in the stator, the motor or electric machine is said to be “synchronous”.
In order to control the torque of such a motor a control system regulates the amplitude of the currents circulating in the stator by applying a suitable sinusoidal voltage to each of the phases of the stator. In order to simplify the control algorithm of the torque in the system, the Park transformation is generally used in order to project the currents and the stator voltages in a rotating reference frame associated with the rotor. Thus, in the Park reference frame the stator voltages to be applied to the stator are determined in order that the corresponding stator currents produce the desired torque. These stator voltages in the Park reference frame are referred to as control signals. By performing the Park inverse transformation the control system then determines the sinusoidal voltages to be applied to the different phases of the stator in order to obtain the desired torque, referred to as the setpoint torque.
The use of such motors, for example as traction motors in an electric or hybrid vehicle, requires a reliable control of the torque that responds quickly depending on the torque needs of the driver and restricts the torque in a value range compatible with the voltage of the traction battery of such a vehicle. When the torque of the motor leaves this value range, the control system of the motor generally becomes unstable, which is to be avoided.
Correctors referred to as integral proportional correctors (IP correctors) are usually used to regulate the currents of the stator of such a motor. Nevertheless, these correctors present problems of instability, in particular when a fast system is desired. In order to overcome this, the parameters of these correctors are calculated in order to ensure the stability margins noted in the specifications to be observed by the motor. In addition, because the intrinsic parameters of permanent magnet motors, such as the internal resistance thereof or inductance thereof, vary from one motor to another, it is known to further increase these stability margins and therefore to further reduce the performance of the corrector so as to be able to use an identical corrector across all motors within the same series of vehicles.
Another example of a corrector not using the integral proportional corrector is disclosed in document EP0702451, which proposes a solution for controlling the speed of a synchronous permanent magnet machine in response to variations in the load of the motor. Such a solution generally requires a calibration of the gain of the corrector as a function of the speed of the machine, or a compensation of static coupling terms, which must be able to be measured perfectly.